Detecting planets outside of the solar system is frequently complicated by the relative proximity of extrasolar planets to the stars they orbit. Since planets can be located only by the reflected light of their star, the star will be significantly brighter than the planet of interest, in some cases on the order of ten million times brighter. In light of the relative proximity of a planet to its star, it is necessary to attenuate the light of the star to obtain a useful image of the planet. In general, this has been accomplished through the use of coronagraphs. The design of coronagraphs can vary, including simple coronagraphs that utilize an occulting disk to block the star's light and nulling coronagraphs that use a phase mask to shift the phase of light, as opposed to directly blocking it. An occulting disk is usually used to block the sun and retrieve corona images. However, it is more suitable to use a phase mask for star systems because the entrance aperture dominates the diffraction effect of images.
In many applications, a given phase mask is designed to interact with light of a certain wavelength. For example, the thickness of a given phase mask can be selected to provide a specific amount of phase to incident light, such that the light of the desired wavelength outputted from the phase mask will have a desired phase pattern. As will be appreciated, light of a different wavelength will be shifted in phase by a different fraction of the wavelength as it passes through the phase mask, skewing the desired phase pattern. Unfortunately, this ensures that a different phase mask is necessary for each wavelength that is of interest, increasing the expense of a coronagraph apparatus, and inconveniencing a user who must replace the phase mask each time an object having different spectral characteristic is viewed.